1. Field of the Invention
The present invention relates generally to display signal processing and display devices for converting a display signal to a pixel voltage, and more particularly to display signal processing and display devices for converting a display signal to a gamma-corrected pixel voltage.
2. Description of the Related Art
A flat-panel display device, which is represented by a liquid crystal display device, is widely used as a display device for a personal computer, a mobile information terminal, a TV, a car navigation system, etc.
In general, the liquid crystal display device comprises a display panel including a matrix array of liquid crystal pixels, and a drive circuit for driving the display panel. A typical display panel has a structure in which a liquid crystal layer is held between an array substrate and a counter-substrate. The array substrate includes a plurality of pixel electrodes that are arrayed in a matrix. The counter-substrate includes a common electrode facing the pixel electrodes. The pixel electrode and the common electrode constitute a liquid crystal pixel together with a pixel region of the liquid crystal layer located therebetween. The alignment state of liquid crystal molecules in the pixel region is controlled by an electric field that is applied between the pixel electrode and the common electrode. In the drive circuit, a digital display signal for each pixel is converted to a pixel voltage by selectively using a predetermined number of reference gradation voltages, and output to the display panel. The pixel voltage is a voltage that is applied to the pixel electrode, with the potential of the common electrode used as a reference.
A conventional reference gradation voltage generating circuit is formed, for example, of ladder resistors that include resistors connected in series between a pair of power terminals and divides the voltage between the power terminals to output a predetermined number of reference gradation voltages (see, e.g. Jpn. Pat. Appln. KOKAI Publication No. 2002-228332).
To express the reproduction characteristic of the liquid crystal display device, a reproduction characteristic curve is used in a graph whose abscissa represents logarithmic values of luminance of a subject itself, such as a scene or a person and ordinate represents logarithmic values of luminance of a reproduction image displayed thereon. When θ represents the inclination angle of the reproduction characteristic curve, tan θ is called “gamma”. Assume that high fidelity is obtainable in the luminance of the reproduction image displayed for the subject, the reproduction characteristic curve forms a linear line whose inclination angle θ is 45°. Since tan 45°=1, the gamma is 1. In other words, the gamma needs to be corrected to 1 in order to obtain high fidelity in the luminance of the reproduction image displayed for the subject. In the above-described reference gradation voltage generating circuit, even if the resistance of the ladder resistor is tuned for gamma correction, it is difficult to make the luminance of the liquid crystal pixel proportional to the gradation value of the display signal.
A technique of gamma correction using reference gradation voltages from a reference gradation voltage generating circuit is conventionally known, for instance, from the disclosure of Jpn. Pat. Appln. KOKAI Publication No. 2001-134242.
In the prior art, however, the same gamma correction is executed for all the three primary colors of red (R), green (G) and blue (B). Consequently, the color balance between red, green and blue is impaired when the luminance of each color is determined as a result of selection from a predetermined number of gradations including black and white. In particular, the luminance of blue after gamma correction is significantly deviated on the side of black, compared to the other colors.